During battery charging it is desirable to deliver a constant current to the battery regardless of battery state of charge or temperature. Prior art circuits to achieve this often require measurement of the current and control of a switching circuit to achieve constant current.
A capacitor charging power supply needs to charge a capacitor from zero voltage to a maximum value. When the capacitor is initially discharged it appears as a short circuit to the charging circuit. Under these conditions the current delivered to it must not exceed the rating of the circuit components.
A constant torque motor is obtained by delivering constant current to the motor irrespective of its speed. A constant torque servo motor can be used for example in robotic applications where the robot grip force can be set by the motor torque limit irrespective of the type of object which is lifted.
Light emitting diodes (LED) and laser diodes also have a requirement to be driven from a constant current supply. The forward voltage of a light emitting diode varies slightly with component tolerances. If a conventional DC to DC power supply was used to drive a range of LEDs without current feedback the current through the LEDs would vary dependent on their forward voltage. Furthermore the forward voltage characteristic varies with temperature. The intensity of the light output would not be constant unless current feedback was added to maintain the current to be constant.
There are therefore many applications where a constant current power supply is desirable. It would be a major advantage if a circuit could be designed to automatically set the current supplied to a load without requiring the complexity of measuring the current and having a closed loop current controlled power supply. Such complexity increases the cost of the power supply circuit. The measurement of the current may require the current to be measured in a part of the circuit which is electrically isolated from the main controller, adding further complexity and cost.
Current measurement circuits employing current sense resistors introduce energy losses which reduce the efficiency of the circuit. Energy efficiency is very important in power supply circuits for all applications. It is particularly important in battery chargers for all applications from portable appliances to electric vehicles. Power supply circuits for LED lighting must be highly efficient as they will be used for long periods of time.
FIG. 1 shows a half bridge inverter circuit connected to an LCL resonant circuit as published in “Simple constant frequency constant current load-resonant power supply under variable load conditions” by H. Pollock, IEE Electronics Letters, Vol. 33. No. 18, 28 Aug. 1997, pp. 1505-1506 and in “Constant frequency, constant current load resonant capacitor charging power supply” by H. Pollock, IEE Proc. Electric Power Applications, Vol. 146, No. 2, March 1999, pp. 187-192. This paper introduced a resonant circuit containing an inductor-capacitor-inductor (T-resonant) arrangement with the load connected in series with the second inductor. The paper reported the discovery that if the two inductors were equal and the circuit was operated at a frequency at which the magnitude of the reactance of the capacitor was also equal to the reactance of the inductors, then the magnitude of the load current was completely independent of the value of the load resistance.
The constant current aspect of the LCL resonant circuit presented in IEE Proc. Electric Power Applications, Vol. 146, No. 2, March 1999, pp. 187-192 was used by Borage, Tiwari and Kotaiah in “Analysis and Design of an LCL-T Resonant Converter as a Constant-Current Power Supply”, published in IEEE Trans. on Industrial Electronics, Vol. 52, No. 6, December 2005, pp. 1547 and further refined by the same authors in “LCL-T Resonant Converter with clamp diodes: A novel Constant-Current Power Supply with inherent constant voltage limit”.
The constant current characteristics of the LCL circuit have been utilised to drive LEDs in “Improvements Relating to Lighting Systems”, WO/2008/120019.
Whilst the LCL resonant converters published in the prior art deliver natural constant current characteristics they have three major disadvantages:                1. The circuits require two high frequency inductors of approximately equal value. The inductors operate at the switching frequency of the circuits which is typically tens of kHz. High frequency inductors suffer from eddy current and hysteresis losses in the cores and skin and proximity effects in the conductors. They are difficult to construct with low losses and since the LCL circuit has two inductors the losses are high.        2. The circuit has a path for dc current to flow through the two inductors and the load. Imbalances of the switch timing between the upper and lower switching transistor in the inverter can cause the mid-point of the dc splitting capacitors to drift substantially from the half the dc supply.        3. The circuit has another resonant frequency, higher than the resonant frequency used for constant current operation. As a result driving the circuit with a square wave voltage at the normal operating frequency will allow harmonics of the fundamental voltage to excite currents close to the higher resonant frequency. As a result distortion of the switch current, away from the pure sinusoidal current desirable for high efficiency, occurs. This distortion causes switching losses and reduces the efficiency of the circuit.        